1. Field of the Invention
This invention relates to a method and apparatus for fixing of spondylolisthesis, e.g., misalignment of the vertebrae comprising the spinal column. More specifically, the present invention relates to an apparatus which is screwed into a drill hole through one vertebra into the adjacent vertebra and a method utilizing that apparatus.
2. Description of the Related Art
Treatments for conditions involving subluxation of one vertebrae upon another, resulting in misalignment of the spinal column, involve the use of screws which extend through a plate and which are tightened to draw the misaligned vertebrae back into alignment. U.S. Pat. No. 5,531,747 to Ray (hereinafter "Ray") is an example of a plate and screw system for treating spondylolisthesis. Other approaches include insertion of an implant into the disk space between misaligned vertebrae such as shown in U.S. Pat. No. 5,601,556 to Pisharodi (hereinafter "Pisharodi"). The Ray approach has its limitations as discussed in Pisharodi. That patent has a disadvantage in requiring removal of the disk and having multiple steps, that is, two implants are inserted into the disk space only to be substituted later with monolithic implants. Henry Bohlman has described a fibular strut used to treat spondylolisthesis with a portion of donated fibular bone in "Spondylolisthesis Treated by a Single-Stage Operation Combing Decompression With In Situ Posterolateral and Anterior Fusion", J. Bone and Joint Surg., 72-A: 415-421, March 1990. This involves obtaining a piece of the fibula of a deceased donor, machining it into the size and shape needed and inserting it into a cavity formed to hopefully make a tight fit.
Many examples of fixation of joints have been described in the literature, including U.S. Pat. No. 4,653,481 to Wiltse (hereinafter "Wiltse"). Many of the approaches involve sophisticated, multi-part systems requiring extensive surgery and many sizes of parts to accommodate different sized patients.
There is, therefore, a need for an improved method of treatment of conditions involving misalignment of the vertebrae of the spinal column, or spondylolisthesis, and it is a principal object of the present invention to provide such a method and an apparatus for use in connection with that method.
The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is "prior art" with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 C.F.R. .sctn. 1.56(a) exists.